The present invention relates to a method and to a device for coulometric determination of water by means of the Karl Fischer reaction in a membrane-free cell.
It is desirable in many cases to be able to determine the amount of water in a gas, a liquid, or in a solid material. This is normal, for instance, in the pharmaceutical industry, the lubricating oil industry, the foodstuff industry, etc. It is also desirable from several aspects to be able to determine the water concentration with the least possible chemical consumption.
The Karl Fischer reaction is well known to the person skilled in this art and, briefly, involves the consumption of water in a reaction with a Karl Fischer reagent that includes, among other things, iodine, sulphur dioxide and a base in a water-free solvent. The amount of water present can be determined, by measuring how much iodine is consumed.
By Karl Fischer reagent is meant here all reagents that satisfy the properties required by the Karl Fischer reaction. Such liquid reagents are well known to the person skilled in this art and will not therefore be described in detail here.
A well known problem with coulometric titration where one and the same Karl Fischer liquid reagent is in connection with both anode and cathode reside in interferences that result from the diffusion/migration of oxidizable reduction products generated in tee cathode reaction into the anode space where they react with the iodine that is generated at the anode and that is intended to react with the water present in the sample. The reduction products cam thus be said to simulate water, wherewith the analysis result becomes too high.
One solution to this problem is to adapt and use in the cathode space a special liquid which will not generate the interfering reduction products. In order to enable two separate liquids to be used in the cell, a membrane is placed between the liquids. In addition to practical handling problems, another drawback with membrane cells is that the membrane is very likely to become clogged, for instance when titrating oils, resulting in current limitations and therewith undesirably long titration times.
In accordance with known titration technology relatively high current pulses, e.g. from 400 mA, must be used when working with present day membrane-free cells. This is because reduction products are not generated to the same high extent at high currents. High currents, however, limit the choice of usable reaction mixtures, meaning that not all types of samples can be analyzed. No solution has yet been proposed in this line of development with respect to the actual problem of handling the reduction products. Instead, the development of membrane-free cells has been concentrated on finding combinations of high currents (for rapid titration) and reagents which generate low concentrations of reduction products at current levels in question.
An example in this regard is found in U.S. Pat. No. 5,300,207 which teaches a high current coulometric titrator for the Karl Fischer determination of water in a membrane-free cell. The iodine generating electrodes are configured such that the cathode has a small extension and the anode has a cupped shape which is open to the cathode, wherein the distance travelled by the electric current is the same from all points on the anode surface to the cathode. This is intended to improve iodine production. Also described is the arrangement of a tube over the cathode for channelling gas generated at the cathode up to the surface of the liquid so as to prevent the gas from interfering with the liquid flow. The tube is described as having holes which ensure circulation of the liquid reagent around the cathode.
An object of the present invention is to overcome the aforesaid drawbacks and to provide a device and a method for the determination of water in a membrane-free cell by means of the Karl Fischer reaction where the user has control over the oxidation of titration-interfering reduction products and is able to regulate such oxidation. Another object of the invention is to provide a device with which titration can be performed even at low currents and with all types of Karl Fischer reagents.
This object is achieved with a device and a method of the kind defined above that have the characteristic set forth in the claims.
It has surprisingly been found that the reliability of the titration results obtained is substantially improved when that part of the liquid reagent located nearest the cathode is prevented from circulating around in the cell together with the remaining liquid reagent.
Because the liquid present around the cathode is stationary, reduction products formed merely represent a local increase in the concentration of said products. They do not then influence the initial titration result. Between two titrations, when the concentration has reached a level at which the risk of migration increases, the cathode liquid can then be mixed with the remaining liquid reagent, wherewith the reduction products are oxidized normally in the course of titration.
The inventive device for coulometric determination of water by means of the Karl Fischer reaction includes a membrane-free measuring cell containing a Karl Fischer type liquid reagent, a means for measuring the level of iodine in the reagent and regulating titration current, and an anode and a cathodic which are spaced from one another and extend down into the liquid reagent, wherein the cathode electrode is disposed in a casing that screens-off in the measuring cell a cathode space between the inner surface of said casing and the cathode, and wherein the casing includes an opening through which the cathode space communicates with the measuring cell liquid reagent that penetrates to a level in the cathode space, and wherein there is arranged in the proximity of the casing a drainage means which when activated causes cathode liquid to flow out through said opening.
The casing may be a tubular element of generally constant internal cross-section and extending down into the liquid reagent with the opening at its free end. The drainage means may be a tillable chamber, an actuatable propeller, or a plunger.
The plunger may have a profile that is complementary to the inner profile of the casing, so as to essentially lie in abutment with the inner surface of the casing The plunger may also be comprised of a shape-pliable material, so that the plunger will be able to seat against the inner walls of the casing irrespective of minor variations in profile.
The cathode may be an electrically conductive wire that had been coated with an electrically non-conductive material and from which the non-conductive material has been removed over a determined surface area of the wire located beneath the liquid surface, so that a constant and small cathode area will be in conductive contact with the liquid reagent, this area being independent of changes in the level of reagent.
The anode may be disposed around the casing. In one embodiment, the anode has the form of a wire wound helically around the casing. In alternative embodiments, the anode may be a braided structure or a net-like structure, e.g. a basin or basket-like structure.
The cathode may be fixedly mounted on the plunger Alternatively, the plunger may be displaceable in relation to the cathode, wherewith in one embodiment the plunger includes a hole through which the cathode wire extends into the cathode space when the plunger is in its retracted position.
The casing wall has provided therein a hole which is located at a distance above the surface of the liquid reagent and downwardly of the plunger when said plunger is in its retracted position, wherewith the liquid level in the cathode space adopts the same level as the surrounding liquid by virtue of pressure equalization, therewith eliminating the risk that hydrogen gas generated by the cathode reaction will fill the cathode space and displace The liquid.
The casing opening located beneath the surface of the liquid will preferably be narrower than the internal cross-sectional area of the casing, therewith further amplifying the screening effect.
The inventive method of coulometrically determining water with a Karl Fischer reaction in a membrane-free cell that includes a liquid reagent, an anode and a cathode and wherein an open cathode space is disposed around the cathode comprises the steps of forcing from the cathode space liquid reagent which contains oxidizable reduction products that have been generated in the cathode reaction and mixing said liquid reagent with remaining liquid reagent, allowing the reduction products to react with iodine in the liquid reagent, again supplying a part of the liquid reagent to the cathode space, and delivering a sample to the liquid reagent located outside the cathode space while taking titration readings in a manner known per se.
Further features of the invention and advantages afforded thereby will be apparent from the following detailed description of a preferred embodiment of the invention, this embodiment being described by way of example only and is not therefore limitive to the scope of the present invention. The various components of the illustrated device have been designated reference numerals to facilitate an understanding of the description.